Epidemiological Characteristics and Trend Forecast of Influenza-like illness in Xuhui District, Shanghai, from 2013 to 2024

Objective To analyze the epidemiological characteristics of influenza-like illness (ILI) in Xuhui District, Shanghai, from 2013 to 2024, and to predict future trends, providing a scientific basis for local influenza prevention and control strategies.Methods Surveillance data on ILI and influenza virus pathogens were collected from sentinel hospitals in Xuhui District from the 14th week of 2013 to the 13th week of 2024. Data processing and statistical analyses were performed using Excel 2016 and SPSS 25.0. Chi-square test, Spearman's correlation analysis and Time series analysis were used to analyze the differences between groups, the relationship between ILI percentage (ILI%) and influenza virus nucleic acid positivity rates, and to predict future epidemic trends.Results From 2013 to 2024, the average ILI% in Xuhui District was 1.01%, with significant annual variations (χ2=45728.241, P<0.001), with a peak in winter, and an additional summer peak observed before the COVID-19 pandemic. The 25-year-old and above group accounted for the highest proportion of ILI cases. A total of 10,882 nasal swab specimens were tested for influenza virus nucleic acids, of which 2,480 (22.79%) were positive, predominantly influenza A (H3N2, 50.04%, 1241/2480). There were significant differences in the positive rates of influenza virus nucleic acid among different surveillance years, seasons and age groups (P<0.001). Influenza activity was highest in winter and spring, with influenza A (H1N1) and influenza B predominating. Before the COVID-19 outbreak, a minor summer peak was observed, primarily driven by influenza A (H3N2). The highest influenza virus positivity rate was observed in the 5-year-old group and below group (36.64%, 107/292). ILI% and influenza virus positivity rate showed a strong positive correlation (r=0.764, P=0.006). The time series model predicts a trough in %ILI in April-June 2025, a small summer peak in July-August, and a winter peak from December through February 2026.Conclusion The predominant influenza strains in Xuhui district varied across surveillance years, with influenza A being the most prevalent type. Influenza exhibited distinct seasonal patterns, with winter-spring peaks, and an additional summer peak before the COVID-19 pandemic. Children under five years old should be a key target group for influenza prevention and control. These findings provide essential insights for refining local influenza prevention strategies.